Band spread tuning system



July 9, 1940.

P. F. G. HOLST El AL BAND SPREAD TUNING SYSTEM Filed Sept. 8, 1938 nuentors 0 Lttorneg P. F. G. HOLST El AL 2 Sheets-Sheet 2 Patented July 9, 1940 Param"I OFFICE BAND SPREAD TUNING SYSTEM Paul F. G. Holst and Loren R. Kirkwood, Oaklyn, N. J., assignors to Radio Corporation of America, acorporation of Delaware Application September 8, 1938, Serial No. 228,892

9 Claims.

The present invention relates to a band spread tuning system for multiple wave bandreceivers of the type having a plurality of tuning inductances tunable by a common variable capacitor to cover a plurality of differing wave bands or frequency ranges such as a broadcast band of 550-1550 kilocycles, and short wave broadcast 49, 3'1, 25 and 19 meter bands, each of which has a range of substantially 300 kilocycles.

band spread tuning system for multiple wave band receivers of the superheterodyne type wherein a plurality of circuits are conjointly tunable by means of a gang or uni-control vari- 151 able capacitor, and has for its primary object to provide an improved band spread tuning system for superheterodyne receivers whereby the tuning indication range for the short wave bands may be made substantially equal to that for the standard broadcast band, without additional variable capacitor sections in the variable tuning capacitor as has heretofore been necessary.

In providing band spread tuning for the short wave bands, full dial scale movement for the tuning indicating means is provided for the short wave bands, as well as for the standard broadcast band, whereby the full dial scale movement For tuning certain of the circuits of a receiver such as the oscillator, for example, a different 35 variable tuning capacitor section is required, having a relatively low maximum capacity as compared with the maximum capacity of the main tuning capacitor for the broadcast band.

It has been found that to maintain approxi- 40 mately a linear scale for the short wave tuning indication a straight line capacity change is desirable in the additional tuning capacitor, whereas for the main tuning capacitor, a capacitor of that type will not produce a straight line frequency characteristic.

For the standard and short wave tuning ranges above referred to, it has been found that the' main variable tuning capacitors for the R. F., detector and oscillator circuits may each have a variable capacity range of approximately 525 mmf., whereas, for the short wave band spread tuning, the oscillator variable capacity section added to the three-unit tuning capacitor, must have a variable capacity range of 15 mmf. to cover the short wave bands with the same dial The invention relates more particularly to a- (Cl. Z50-40) .scale tuning indication or band spread as the standard broadcast band.

The use of an additional capacitor section or sections in a variable tuning capacitor not only increases the cost and the size of the unit, but' also involves the use of additional selector switching means for connecting the various sections in circuit. It is, therefore, an object of the present invention to provide an improved band spread tuning system including a capacitor 10 network which eliminates the necessity for utilizing an additional tuning capacitor section in a multiple wave band receiver.

Itis a still further object of the present in vention to provide an improved tuning system for multiple wave band receivers embodyinga variable tuning capacitor adapted for. tuning through a predetermined frequency range such as the broadcast band, which includes means for modifying a circuit in connection with said capacitor whereby its tuning characteristic and tuning range are adapted for band spread tuning in higher frequency ranges of lesser kilocycle coverage, without resorting to the use of additional variable tuning capacitor sections in conjunction therewith.

The invention will, however, be better understood from the following description when considered in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

In the drawings Figure l is a diagrammatic representation of a radio receiving system provided with a tuning system embodying the invention;

Figure 2 is a schematic circuit diagram of a portion of the receiving system shown in Fig. 1 showing details of the switching and tuning cir cuits included therein;

Figure 3-is a schematic circuit diagram of a 40 `portion of Fig. 1 with the circuit simplified to indicate the standard broadcast band tuning connections; and

Figure 4 is a similar simplified schematic circuit diagram of a portion of Fig. 1 showing the circuits provided therein for tuning one of the short wave bands such as the 31 meter band.

Referring to Fig. l, the R. F. detector and oscillator sections 5, 6 and 1, respectively, of a gang variable tuning capacitor for a radio receiving system are shown in connection with atuning system 8 provided with a wave band change means 9 and a variable tuning control means I0 connected as indicated by the dotted connection I I with the variable tuning capacitor. 55

The tuning indicator means l2 in connection with the tuning control means lil is provided with a plurality of indicator scales, two of which are indicated at i3 and lli,- and the scales are provided with a common movable indicator I5. The scales shown may be considered to be for the standard broadcast and one of the short wave bands such as the 31 meter band.

In operation, the wave band change means 9 controls connections with the variable tuning capacitors whereby the same width or range of tuning indication movement of the indicator i5 is provided for the short wave band or bands as for the broadcast band, without the addition of avariable capacitor or capacitors for the short Wave band. The circuits and means for effecting this change are more fully shown in Fig. 2, to which attention is now directed along with Fig. l.

In Fig. 2 only the oscillator circuits and switch means are shown since for the spread band tuning, the R. F and detector tuning capacitors 5 and ii are rendered ineffective and the circuits associated therewith are tuned to substantially the middle of each band to be covered and signal selection is obtained by varying the oscillator tuning.

The oscillator` tube is indicated at 2Q and coniprises a cathode 2l, a control grid 22, a screen grid 23, a suppressor grid an output anode 25. The oscillator is tunable by means of the variable capacitor section 'l which is of the logarithmic or modified straight line frequency type in connection with tuning inductances indicated at 25e-3l for the broadcast or A band, the police and amateur or B band, the short wave or C band, and the various international entertainment broadcast bands in the C band comprising the 49, 31, 25 and 19 meter bands, all as indicated adjacent the inductances referred to.

The oscillator grid 22 and the variable tuning capacitor 'l are selectively connectable with the various inductances through the medium of a selector switch from a part of the wave band change means, of which two sections 32 and 33 for the oscillator are shown in Fig. 2, the remainder of the sections being eliminated in the drawing for the sake of simplifying the same as the R. F. and detector circuits may be arranged in any suitable manner to provide the tuning arrangement hereinbefore referred to, one arrangement of that character being shown, described and claimed in an application of John D. Reid, Serial No. 139,847, filed April 30, 1937, for Multiple wave band receivers, and assigned to the same assignee as this application.

.The general circuit arrangement is such that the cathode 2l of the oscillator is connected to a tap indicated at on the 49 meter and B band inductance 2l which is connected at one end to the oscillator output lead Sii. The supply circuit for the anode 25 and the screen grid 23 is provided with by-pass capacitors S5 and 5f. The oscillator circuit for the A band furthermore includes the feedback winding 39 and a portion oi the winding 2T which provides the feedback for all additional Wave bands. An additional feedback for the 19 meter band from the anode 25 is provided through a winding 5B. The windings 26, 21 and 23 are tunable by means of the variable capacitor l operating through its full capacity range of substantially 1l to 536 mmf. in conjunction with series tracking capacitors al, 42 and 43. For the band spread tuning, the in- -ductance 2'! alone and the inductances 29, Sil

and 3i successively connected in shunt with the inductance 27 are tuned by the capacitor 'l' operating through a limited capacity range of substantially 15 mmf, in connection with a shunt capacitor l5 and the series tracking capacitor 33.

The switch sections 32 and 33 are shown in a position providing connections for the A. or standard broadcast band tuning and rotate clockwise through the B, C, 49, 3l, 25 and i9 meter positions in the order named. Since the invention is concerned primarily with the switching operations in connection with the variable tuning capacitor whereby its tuning characteristic and capacity range are controlled, the standard broadcast and one band spread range are herein considered and the connections therefor in simplied form are shown in Figs. 3 and 4, to which attention is now directed along with Fig. 2.

The input grid 22 is provided with a grid resistor it and grid coupling capacitor ll through which it is connected witha contact #iii of the switch section 32 and with the serios tracking capacitor 53. The series tracking capacitor l2 is placed in parallel with the capacitor i3 through a movable contact element I9 in the switch section 32 between the contact d8 and a contact 59. A through connection between the switch sections 32 and indicated by thc dotted line 5l, provides connection with a movable contact element 52 in the switch section 33 and through a xed contact 53 and a capacitor 55 provides coupling with the high potential end 5d of the A band inductance 26, as shown more clearlyv in Fig. 3.

The capacitors indicated at 56, 5l and 58 are also included in an automatic frequency control circuit for the A band tuning and do not concern the present invention, the capacitor 56 being substantially .05 mf. and eiiectively grounding the low potential end of the winding 2S to the tuning capacitor 'l in the circuit at present considered. For tracking with the tuning capacitor sections 5 and 6 of Fig. 1, the capacitor I is provided with series tracking capacitors i2 and d3 which may have, respectively, capacities oi 560 mmf. and 62 mmf.

Rotating the selector switch means il four positions clockwise provides the connection shown in Fig. 4, in which the 31 meter inductancc 29 is placed in shunt with the 49 meter inductance 21, the larger series tracking capacitor 42 is removed from circuit, and the shunt capacitor .45 is placed in parallel with the tuning capacitor This may have a capacity oi substantially mmf. This arrangement permits the capacitor 13 to remain in circuit for both the main and band spread tuning ranges.

Two additional capacitors Si! are also connected in parallel with the coils 29 as shown in Fig. 4. The output from the oscillator to the iirst detector 62 is indicated by the connection lead 63 and is applied to the oscillation input grid 64 as shown in Fig. 3. One of the capacitors 6D is of the negative temperature coefficient type in order to compensate for changes in other parts of the circuit which would result in oscillator drift while the receiver is heating up.

With a variable tuned capacitor 'l used for variable tuning of the main tuning bands such as the standard broadcast band and having a variable capacity range of substantially 525 mmf. may be used for band spread tuning with a capacity range of substantially 1 5 mmf. by reducing the series tracking capacitor and increasing the shunt capacity in connection with the capacitor l. `The capacitors 43 and 45 in series and in shimt, respectively, with the capacitor 'l in the capacitor network are so proportioned that the capacitychange of the combination is substantially straight line capacity in character and permits substantially 50 percent of the capacity -change to be obtained at 50 percent rotation or movement of the tuning capacitor l. This is the same range as formerly obtained with an additional smaller variable capacitor providing the same characteristic, and is less than ten percent of the normal capacity variation range.

Without series and shunt capacitors, the variable capacitor 1 may have a minimum capacity of 11 mmf., a capacity of substantially 125 mmf. at

50 percent rotation, and a capacity of 536 mmf. at 100 percent rotation.

To cover the 300 kc. spaced bands hereinbefore referredto, the series tracking capacitor 43 is in the order oi 10 percent of the maximum capacity of the variable capacitor 1, while the additional tracking capacities provided by the capacitor i2 is of the order of the maximum capacity of the capacitor 1. 'Ihe shunt capacitor 45 is also of the order of twice the capacity of the series tracking capacitor 43. The circuit provided as in Figs. 3 and 4 and controlled by the switch means shown in Fig. 2 provides for substantially linear scales for the dial scales i3 and I4 of Fig. l for the tuning in the standard broadcast or the band spread bands and effectively eliminates the necessity for an additional variable tuning capacitor section on the main variable tuning capacitor of the radio receiving system, thereby eliminating the additional cost of the same and additional switching of the capacitor for placing it in circuit.

While the invention has been described with particular reference to the oscillator circuit, it will be appreciated that it may be applied to other tunable circuits in a radio receiver. It is, however, particularly adapted for applying band spread tuning to a superheterodyne receiver in connection with the oscillator, and the circuit shown and described represents a present preferred embodiment of the invention.

We claim as our invention:

l. A band spread tuning system comprising in combination, a variable tuning capacitor having a plurality of variable capacitor sections of predetermined relatively wide capacity variation range and a predetermined tuning characteristic, and means for altering the capacity variation range and tuning characteristic of at least one of said capacitor sections whereby it provides substantially straight line capacity variation over a relatively limited 'capacity variation range.

2. A band spread tuning system comprising in combination, a variable tuning capacitor having a plurality of variable capacitor sections of predetermined relatively wide capacity variation range and a predetermined tuning characteristic, and means for altering the capacity variation range and tuning characteristic of at least one of said capacitor sections whereby it provides substantially straight line capacity variation over a relatively limited capacity variation range, said last named means comprising a capacitor in series and a capacitor in shunt with said variable capacitor section providing therewith a capacitor networks, and means providing an additional capacitor in shunt with said series capacitor and for disconnecting said first named shunt capacitor having capacities so proportioned that the i'irst named capacity variation range and tuning characteristic substantially is restored.

3. In a radio receiving system, tuning means comprising a variable tuning capacitor having a plurality of variable capacitor sections ofv substantially equal capacity variation range and tuning characteristic, and means for altering the capacity variation range andtuning characteristic of at least one of said capacitor sections whereby it provides substantially straight line capacity variation over a relatively limited capacity variationrange for band spread tuning.

4. In a radio receiving system, tuning means comprising a variable tuning capacitor having a plurality of variable capacitor sections of substantially equal capacity variation range and tuning characteristic, means for altering the capacity variation range and tuning characteristicof at least one of said capacitor sections whereby it provides substantially straight line capacity variation over a relatively limited capacity variation range for band spread tuning, a plurality of inductance elements, means providing a plurality of tunable circuits in connection with each of said capacitor sections, and means conjointly operable with said second named means for selectively connecting said inductance elements in circuit with said one of said capacitor sections for band spread tuning in a plurality of wave bands. n

5. A band spread tuning system comprising a variable tuningl capacitor having a plurality of variable capacitor sections of predetermined capacity variation range and tuning characteristic, vmeans providing a capacitor network for altering the capacity variation range and tuning characteristic of at least one of said capacitor sections for band spread tuning in a relatively narrow short wave tuning band, said capacitor network including a capacitor in series and a capacitor in shunt with said one capacitor section whereby it provides substantially straight line capacity variation over a relatively limited capacity variation range, and means providing a second capacitor in parallel with said series capacitor and for disconnecting said shunt capacitor for tuning in a relatively wide lower frequency tuning band.

6. A band spread tuning system comprising a variable tuning capacitor having a plurality of variable capacitor sections of predetermined capacity variation range and tuning characteristic,

means providing a capacitor network for altering the capacity variation range and tuning characteristic of at least one of said capacitor sections whereby it provides substantially straight line capacity variation over a relatively limited capacity variation range, said capacitor network including a capacitor in series and a capacitor in shunt with said one capacitor section so proportioned with respect to the capacity of said variable capacitor section that at substantially 50 percent of the variable tuning movement of said capacitor section substantially 50 percent of the modied capacity variation range is effected.

7. In a radio receiving system, tuning means comprising a variable tuning capacitor having a plurality of variable capacity sections of substantially equal capacity range and predetermined capacity variation range, means providing a tunable signal circuit in connection with each of said capacitor sections, means providing a plurality of inductance elements for said circuits, wave band change means for selectively connecting said inductance elements with said circuits,

means providing a series tracking capacityfor one of said capacitor sections, and means conjointly operable with said wave band change means for reducing the value of the series trackingcapacity and providing a capacity in shunt with said variable tuning capacitor section so proportioned that the capacity change of the capacitor network is substantially straight line capacity in character for band spread tuning one of said circuits in a limited frequency range.

8. In a radio receiving system, tuning means comprising a variable tuning capacitor having a plurality of variable capacitor sections of substantially equal capacity range and predetermined varia-ble capacity range, and means for altering the capacity range and tuning characteristic of at least one of said capacitor sections whereby it provides substantially straight line capacity variation over a capacity variation range less than ten per cent of the rst named variable capacity range for band spread tuning.

9. In a superheterodyne radio receiver, the combination of an oscillator tube, a plurality of selectable oscillator tuning inductances, means for selectively connecting said inductances in circuit vvith said tube for generating oscillations in differing frequency ranges, a variable capacitor connected in circuit for tuning each of said inductances, means providing a series tracking capacity for said variable tuning capacitor, means for reducing the capacity Value of said series tracking capacity below the maximum capacity of said variable capacitor and providing a shunt capacitor of predetermined capacity value for said variable capacitor, said capacity values being so proportioned that substantially straight line capacity Variation in tuning is eiected by said variable capacitor, with substantially linear tuning in at least two diiering frequency ranges.

PAUL F. G. HOLST. LOREN R. KIRKWOOD. 

